Light-absorbing material, method for producing the same, and solar cell including the same

ABSTRACT

A light-absorbing material includes a compound, wherein the compound has a perovskite crystal structure represented by the formula AMX 3  where a Cs +  ion is located at an A-site, a Ge 2+  ion is located at an M-site, and I −  ions are located at X-sites, and at least a part of the compound has an orthorhombic perovskite crystal structure. An X-ray diffraction pattern of the compound measured using Cu Kα radiation may have a first peak at a diffraction angle (2θ) of 25.4° or more and 25.8° or less and a second peak at a diffraction angle (2θ) of 24.9° or more and 25.3° or less, and an intensity of the first peak may be 30% or more of an intensity of the second peak.

BACKGROUND 1. Technical Field

The present disclosure relates to a light-absorbing material having aperovskite crystal structure, a method for producing the light-absorbingmaterial, and a solar cell containing the light-absorbing material.

2. Description of the Related Art

In recent years, solar cells containing a compound (hereinafter referredto as “perovskite-type compound”) having a perovskite crystal structurerepresented by the formula AMX₃ or a similar crystal structure as alight-absorbing material have been being developed. In theperovskite-type compound, a monovalent cation is located at an A-site, adivalent metal cation is located at an M-site, and halogen anions arelocated at X-sites.

A perovskite-type compound-containing solar cell (hereinafter referredto as “perovskite solar cell”) is one of candidates for low-cost,high-efficiency next-generation solar cells and is being developed.

From the viewpoint of environmental regulation, the following compoundis known as a lead-free perovskite-type compound: a perovskite-typecompound in which a Cs⁺ ion (cesium cation) is located at an A-site, aGe²⁺ ion (germanium cation) is located at an M-site, and I⁻ ions arelocated at X-sites (see, for example, Chinese Examined PatentApplication Publication No. 103943368 and Thirumal Krishnamoorthy etal., Journal of Materials Chemistry A, vol. 3 (October, 2015), pp.23829-23832 (hereinafter referred to as “Non-patent Document 1”)).

SUMMARY

In order to increase the conversion efficiency of perovskite solarcells, a light-absorbing material capable of achieving high conversionefficiency at sunlight wavelengths is demanded.

One non-limiting and exemplary embodiment provides a light-absorbingmaterial which contains no lead and which is capable of achieving highconversion efficiency at sunlight wavelengths.

In one general aspect, the techniques disclosed here feature alight-absorbing material containing a compound. The compound has aperovskite crystal structure represented by the formula AMX₃ where a Cs⁺ion is located at an A-site, a Ge²⁺ ion is located at an M-site, and I⁻ions are located at X-sites. At least a part of the compound has anorthorhombic perovskite crystal structure.

It should be noted that general or specific embodiments may beimplemented as an element, a device, a module, a system, an integratedcircuit, or a method. It should be noted that the general or specificembodiments may be implemented as any selective combination of anelement, a device, a module, a system, an integrated circuit, and amethod.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a solar cell containing alight-absorbing material according to an embodiment of the presentdisclosure;

FIG. 2 is a schematic sectional view of another solar cell containingthe light-absorbing material;

FIG. 3 is a graph showing an absorption spectrum of a compound of eachof Example 1 and Comparative Example 1; and

FIG. 4 is a graph showing an X-ray diffraction pattern of a compound ofeach of Example 1 and Comparative Example 1.

DETAILED DESCRIPTION

Underlying knowledge forming the basis of the present disclosure is asdescribed below.

It is known that the performance of a light-absorbing material for solarcells depends on the band gap thereof. Details are described in WilliamShockley et al., Journal of Applied Physics, vol. 32, no. 3 (March,1961), pp. 510-519. The limit of conversion efficiency is known as theShockley-Queisser limit. When the band gap of the light-absorbingmaterial is 1.4 eV, the theoretical conversion efficiency thereof ishighest. When the band gap of the light-absorbing material is largerthan 1.4 eV, a high open-circuit voltage is obtained and theshort-circuit current is, however, reduced by a reduction in absorptionwavelength. In contrast, when the band gap of the light-absorbingmaterial is smaller than 1.4 eV, the short-circuit current is increasedby an increase in absorption wavelength and the open-circuit voltage is,however, reduced.

The band gap of a perovskite-type compound (CsGeI₃) described inNon-patent Document 1 is about 1.6 eV and is unequal to 1.4 eV, at whichthe theoretical conversion efficiency thereof peaks. When theperovskite-type compound is contained in a solar cell, there is aproblem in that high conversion efficiency is not obtained because lightwith a wavelength of more than 780 nm cannot be used.

The inventor has investigated a process for synthesizing CsGeI₃ and thecrystal structure of CsGeI₃. As a result, the inventor has found thatCsGeI₃ described in Non-patent Document is rhombohedral and can bereduced in band gap by converting at least a part of CsGeI₃ into anorthorhombic crystal. On the basis of this finding, the inventor hasfound a light-absorbing material capable of achieving higher conversionefficiency than ever before at long wavelengths, thereby completing thepresent disclosure.

The present disclosure is based on the above findings and is assummarized below.

A light-absorbing material according to an aspect of the presentdisclosure includes a compound. The compound has a perovskite crystalstructure represented by the formula AMX₃ where a Cs⁺ ion is located atan A-site, a Ge²⁺ ion is located at an M-site, and I⁻ ions are locatedat X-sites. At least a part of the compound has an orthorhombicperovskite crystal structure.

An X-ray diffraction pattern of the compound measured using Cu Kαradiation may have a first peak at a diffraction angle (2θ) of 25.4° ormore and 25.8° or less and a second peak at a diffraction angle (2θ) of24.9° or more and 25.3° or less. An intensity of the first peak may be30% or more of an intensity of the second peak.

The compound may have a band gap of, for example, 1.35 eV or more and1.53 eV or less.

The compound may have a band gap of, for example, 1.45 eV or more and1.48 eV or less.

A solar cell according to an aspect of the present disclosure includes:a first electrode having electrical conductivity; a second electrodehaving electrical conductivity; and a light-absorbing layer between thefirst electrode and the second electrode, the light-absorbing layerconverting incident light into electric charge. The light-absorbinglayer contains the light-absorbing material.

A method for producing a light-absorbing material according to an aspectof the present disclosure, the light-absorbing material including acompound having a perovskite crystal structure represented by theformula AMX₃ where a Cs⁺ ion is located at an A-site, a Ge²⁺ ion islocated at an M-site, and I⁻ ions are located at X-sites. The methodincludes: (A) preparing a first solution containing Ge²⁺ ions and I⁻ions; (B) preparing a second solution containing Cs⁺ ions; and (C)precipitating the compound by introducing the second solution into thefirst solution adjusted to a predetermined temperature.

The (B) may include preparing the second solution by dissolving, forexample, a Cs source in an organic solvent.

The (A) may include preparing the first solution by mixing, for example,a Ge source, hydriodic acid, and phosphinic acid.

The Ge source may be, for example, germanium diiodide. The predeterminedtemperature may be, for example, 50° C. or more and 100° C. or less.

The Ge source may be, for example, germanium oxide. The predeterminedtemperature may be, for example, 20° C. or more and 50° C. or less.

The (A) may include preparing, for example, a third solution bydissolving the Ge source in an organic solvent and preparing the firstsolution by mixing the third solution with a liquid mixture containingthe hydriodic acid and the phosphinic acid.

Embodiments

A light-absorbing material according to an embodiment of the presentdisclosure contains a perovskite-type compound. The perovskite-typecompound in this embodiment contains a compound which has a perovskitecrystal structure represented by the formula AMX₃ where a Cs⁺ ion islocated at an A-site, a Ge²⁺ ion is located at an M-site, and I⁻ ionsare located at X-sites.

An X-ray diffraction pattern of the perovskite-type compound measuredusing Cu Kα radiation in this embodiment has a first peak located at adiffraction angle (2θ) of 25.4° or more and 25.8° or less and a secondpeak located at a diffraction angle (2θ) of 24.9° or more and 25.3° orless. The first peak is assigned to the (111) plane of orthorhombicCsGeI₃ and is a peak detected when CsGeI₃ which is an orthorhombicperovskite-type compound is contained. The second peak is assigned tothe (111) plane of rhombohedral CsGeI₃ and is a peak detected whenCsGeI₃ which is a rhombohedral perovskite-type compound is contained.That is, the perovskite-type compound in this embodiment is in such astate that orthorhombic CsGeI₃ and rhombohedral CsGeI₃ are mixed.

As described above, a perovskite-type compound, CsGeI₃, described inNon-patent Document 1 is rhombohedral and has a band gap of about 1.60eV. On the other hand, the perovskite-type compound in this embodimentcontains orthorhombic CsGeI₃ in addition to rhombohedral CsGeI₃. Thisallows the perovskite-type compound in this embodiment to have a bandgap smaller than that of a perovskite-type compound composed of onlyrhombohedral CsGeI₃.

The proportion of the intensity of the first peak to the intensity ofthe second peak is, for example, 30% or more. The term “intensity ofpeak” as used herein refers to the maximum of a peak. The fact that theintensity of the first peak is 30% or more of the intensity of thesecond peak allows the band gap to be sufficiently small. The intensityof the first peak may be 60% or more of the intensity of the secondpeak. The upper limit of the proportion of the intensity of the firstpeak to the intensity of the second peak is not particularly limited.The intensity of the first peak may be greater than the intensity of thesecond peak. In the case of producing a perovskite-type compound by amethod below, the intensity of a first peak of an X-ray diffractionpattern of the perovskite-type compound may be, for example, 30% or moreand 100% or less of that of a second peak thereof.

According to investigations carried out by the inventors, as theproportion of orthorhombic CsGeI₃ in a perovskite-type compound ishigher, a smaller band gap can be obtained. Thus, most of theperovskite-type compound in this embodiment may be orthorhombic CsGeI₃.Alternatively, the perovskite-type compound in this embodiment may besubstantially composed of orthorhombic CsGeI₃ only. When theperovskite-type compound in this embodiment is composed of orthorhombicCsGeI₃ only, an X-ray diffraction pattern of the perovskite-typecompound has no second peak. Such a perovskite-type compound can beproduced in such a manner that, for example, orthorhombic CsGeI₃ istaken from a perovskite-type compound produced by a method below and isthen grown.

The band gap of the perovskite-type compound in this embodiment isdesirably, for example, 1.30 eV or more and less than 1.60 eV, moredesirably 1.35 eV or more and 1.53 eV or less, and further moredesirably 1.45 eV or more and 1.48 eV or less. The magnitude of the bandgap thereof may vary depending on a method and conditions for producingthe perovskite-type compound.

The perovskite-type compound in this embodiment may mainly containCsGeI₃ and may further contain a trace amount of another element. CsGeI₃contains a Cs⁺ ion as a cation located at an A-site and may furthercontain, for example, 5 mol % or less of another cation. Examples of theother cation include organic cations such as a CH₃NH₃ ⁺ ion and a(NH₂)₂CH⁺ ion and alkali metal cations such as an Rb⁺ ion. CsGeI₃contains I⁻ ions as halogen anions located at X-sites and may furthercontain, for example, 5 mol % or less of another halogen anion such as aBr⁻ ion or a Cl⁻ ion.

Method for Producing Light-Absorbing Material

An example of a method for producing the light-absorbing materialaccording to this embodiment is described below.

First, a Ge-containing solution containing a Ge source, hydriodic acid,and phosphinic acid (reducing agent) is prepared. The Ge source may be amaterial soluble in a liquid mixture of hydriodic acid and phosphinicacid. The examples of the Ge source may include germanium diiodide(GeI₂), germanium tetraiodide (GeI₄), and germanium oxide (GeO₂). The Gesource solution is obtained in such a manner that, for example, the Gesource or a Ge source solution containing the Ge source is introducedinto the liquid mixture of hydriodic acid and phosphinic acid, followedby heating. The heating temperature may be, for example, 50° C. or moreand 100° C. or less. The Ge-containing solution may be a solutioncontaining Ge²⁺ ions and I⁻ ions. A method for preparing theGe-containing solution is not limited to the above.

The Ge source solution is prepared by dissolving the Ge source in anorganic solvent. The examples of the organic solvent may includedimethyl sulfoxide, and N,N-dimethylformamide.

A Cs source solution containing Cs⁺ ions is prepared. The Cs sourcesolution is prepared, for example, by dissolving the Cs source in anorganic solvent. The Cs source may be a material soluble in the organicsolvent. The examples of the Cs source may include cesium iodide (CsI).The examples of the organic solvent may include, for example, dimethylsulfoxide, and N,N-dimethylformamide.

Next, the temperature of the Ge-containing solution is adjusted to afirst temperature, followed by introducing the Cs source solution intothe Ge-containing solution. A mixture of the Ge-containing solution andthe Cs source solution may be maintained at the first temperature asrequired. This precipitates a perovskite-type compound (CsGeI₃) in themixture thereof.

Controlling the temperature of the Ge-containing solution to the firsttemperature before the Cs source solution is introduced into theGe-containing solution allows CsGeI₃ to be obtained in such a state thatan orthorhombic crystal and a rhombohedral crystal are mixed. Thedesirable range of the first temperature varies depending on the Gesource. When germanium diiodide (GeI₂) is used as the Ge source, thefirst temperature may be 50° C. or more and 100° C. or less. Whengermanium oxide (GeO₂) is used as the Ge source, the first temperaturemay be 20° C. (room temperature) or more and 50° C. or less. Controllingthe temperature of the Ge-containing solution within the above rangeenables the rate of reaction to be adjusted within such a range thatCsGeI₃ can be obtained in such a state that the orthorhombic crystal andthe rhombohedral crystal are mixed.

In a method for producing CsGeI₃ described in Non-patent Document 1, aGe source (GeO₂) in a powder state and a Cs source (CsI) in a powderstate are dissolved in the Ge-containing solution in that order.According to this method, the rate of reaction is high because powderysources are mixed. When the rate of reaction is high, a material with ahigher production rate is preferentially produced. That is, it isconceivable that rhombohedral CsGeI₃, which is more likely to beproduced than orthorhombic CsGeI₃, is mainly formed. However, in thisembodiment, a solution is prepared by dissolving the Cs source in theorganic solvent and is then mixed with the Ge-containing solution.Therefore, the rate of reaction can be suppressed to a low level; hence,not only rhombohedral CsGeI₃ but also orthorhombic CsGeI₃ are produced.

The method for producing the light-absorbing material is not limited tothe above-mentioned method. A perovskite-type compound containingrhombohedral CsGeI₃ and orthorhombic CsGeI₃ can be precipitated bymixing Ge²⁺ ions, I⁻ ions, and Cs⁺ ions under such conditions that therate of reaction is low.

Solar Cell

A solar cell 100 containing the light-absorbing material according to anembodiment of the present disclosure is described below.

FIG. 1 is a schematic sectional view of the solar cell 100, whichcontains the light-absorbing material according to an embodiment of thepresent disclosure.

The solar cell 100 includes a substrate 101, a first electrode 103, alight-absorbing layer 102, and a second electrode 104, the firstelectrode 103, the light-absorbing layer 102, and the second electrode104 being stacked on the substrate 101 in that order.

The light-absorbing layer 102 is a layer converting incident light intoelectric charge. The light-absorbing layer 102 contains thelight-absorbing material.

In the solar cell 100, when light is applied to the light-absorbinglayer 102 from the outside, the light-absorbing layer 102 absorbs lightto generate electrons and holes. The electrons and holes generated inthe light-absorbing layer 102 are output to the outside through thefirst electrode 103 and the second electrode 104, respectively.

The substrate 101 has a function to physically support thelight-absorbing layer 102, the first electrode 103, and the secondelectrode 104. The substrate 101 may be made of, for example, atransparent material or an opaque material. Examples of the transparentmaterial include glass and transparent plastics. Examples of the opaquematerial include metals, ceramics, and opaque plastics. When thesubstrate 101 is made of the transparent material, electricity can beobtained by irradiating the light-absorbing layer 102 with sunlightpassing through the substrate 101.

When one or both of the first electrode 103 and the second electrode 104have sufficient strength, the substrate 101 can be omitted. Referring toFIG. 1, the substrate 101 is placed in contact with the first electrode103. The substrate 101 may be placed in contact with the secondelectrode 104.

The first electrode 103 and the second electrode 104 may be made of anelectrically conductive material. Examples of the electricallyconductive material include metals, transparent metal oxides, and carbonmaterials. Examples of the metals include gold, silver, copper,platinum, aluminium, titanium, nickel, tin, zinc, and chromium. Examplesof the transparent metal oxides include indium-tin composite oxides,antimony-doped tin oxides, fluorine-doped tin oxides, and zinc oxidesdoped with boron, aluminium, gallium, and/or indium. Examples of thecarbon materials include graphenes, carbon nanotubes, and graphite.

One or both of the first electrode 103 and the second electrode 104desirably have light transmissivity in the visible-to-near infraredrange. Even when either one of the first electrode 103 and the secondelectrode 104 is made of an opaque material such as metal or a carbonmaterial, light transmissivity can be ensured by forming a pattern fortransmitting light. The pattern for transmitting light may be a grid,linear, or wavy pattern.

When the first electrode 103 and the second electrode 104 have lighttransmissivity, the transmittance thereof is desirably high. Thetransmittance thereof is, for example, 50% or more and is desirably 80%or more. The wavelength range of transmitted light is desirably widerthan the absorption wavelength range of the light-absorbing material,which is contained in the light-absorbing layer 102.

FIG. 2 is a schematic sectional view of another solar cell 200containing the light-absorbing material.

As shown in FIG. 2, an electron transport layer 105 may be placedbetween a light-absorbing layer 102 and a first electrode 103. Thepresence of the electron transport layer 105 enables the efficiency ofextracting electrons from the first electrode 103 to be increased. Theelectron transport layer 105 is typically composed of a semiconductormaterial.

Examples of the semiconductor material used in the electron transportlayer 105 include metal oxide materials and organic n-type semiconductormaterials. Examples of the metal oxide materials include titanium oxide,tin oxide, zinc oxide, and indium oxide. Examples of the organic n-typesemiconductor materials include imide compounds, quinone compounds,fullerenes, and derivatives thereof.

A hole transport layer 106 may be placed between the light-absorbinglayer 102 and a second electrode 104. The presence of the hole transportlayer 106 enables the efficiency of extracting holes from the secondelectrode 104 to be increased. The hole transport layer 106 is typicallycomposed of a semiconductor material.

Examples of the semiconductor material used in the hole transport layer106 include inorganic p-type semiconductor materials and organic p-typesemiconductor materials. Examples of the inorganic p-type semiconductormaterials include CuO, Cu₂O, CuSCN, molybdenum oxide, and nickel oxide.Examples of the organic p-type semiconductor materials includephenylamine, triphenylamine derivatives containing a tertiary amine inthe framework thereof, and PEDOT compounds having a thiophene structure.

The solar cell 100 can be manufactured by, for example, a method below.

First, the first electrode 103 is formed on the substrate 101. Aphysical vapor deposition process or a chemical vapor deposition processcan be used to form the first electrode 103. Examples of the physicalvapor deposition process include a sputtering process, a resistiveheating evaporation process, and an electron beam evaporation process.Examples of the chemical vapor deposition process include a thermalchemical vapor deposition process, a plasma-enhanced chemical vapordeposition process, and an atomic layer deposition process.

Next, the light-absorbing layer 102 is formed on the first electrode103. The light-absorbing layer 102 is formed by coating the firstelectrode 103 with a liquid (coating solution) containing thelight-absorbing material. The coating solution can be prepared bymixing, for example, the perovskite-type compound (CsGeI₃) prepared bythe above-mentioned method with a solvent. The solvent may be one thatdoes not decompose the perovskite-type compound CsGeI₃. Examples of asolvent that does not decompose CsGeI₃ include toluene and water.Examples of a coating process include a spin coating process, a diecoating process, an ink jet process, and a blade coating process.

Subsequently, the second electrode 104 is formed on the light-absorbinglayer 102. A technique similar to a process for preparing the firstelectrode 103 can be used to prepare the second electrode 104. A processthat does not damage the light-absorbing layer 102 may be appropriatelyselected from the above processes.

Example

A compound of Example 1 and a compound of Comparative Example 1 wereprepared and were examined for crystal structure and band gap. InExample 1, a perovskite-type compound (CsGeI₃) was prepared by mixing asolution of a Cs source with a Ge-containing solution. In ComparativeExample 1, a perovskite-type compound was prepared by mixing a powder ofthe Cs source with the Ge-containing solution, similarly to a methoddescribed in Non-patent Document 1.

A method for preparing the compound of Example 1, a method for preparingthe compound of Comparative Example 1, and analysis results thereof aredescribed below.

Method for Preparing Compound of Example 1

First, germanium diiodide (GeI₂) was dissolved in a solvent prepared bymixing N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at aratio of 4:1 so as to give a concentration of 1 mol/L, whereby a Gesource solution was obtained.

Furthermore, a Cs source solution was prepared in such a manner thatcesium iodide (CsI) was dissolved in dimethyl sulfoxide (DMSO) so as togive a concentration of 1 mol/L.

Next, the Ge source solution, a hydriodic acid solution (a concentrationof 55%), and a phosphinic acid solution (a concentration of 33%) weremixed at a volume ratio of 4:4:1. The mixture was heated to 100° C. witha hotplate, whereby the Ge-containing solution was prepared.

Subsequently, the Cs source solution was added dropwise to theGe-containing solution heated to 100° C. The mixing ratio of the Cssource solution to the Ge-containing solution was 1:9. After the Cssource solution was added dropwise to the Ge-containing solution, thismixture was left for about 15 minutes in such a state that this mixturewas heated at 100° C., whereby black particles were precipitated in aliquid. The particles were washed with ethanol and were then dried,whereby the compound of Example 1 was obtained.

Method for Preparing Compound of Comparative Example 1

A germanium oxide (GeO₂) powder was dissolved in a mixture of ahydriodic acid solution (a concentration of 55%) and a phosphinic acidsolution (a concentration of 33%) so as to give a concentration of 0.125mol/L. This solution was left in such a state that this solution washeated to 100° C. with a hotplate, whereby a yellow liquid(Ge-containing solution) was obtained.

Next, a powder of cesium iodide (CsI) was mixed with the Ge-containingsolution heated to 100° C. so as to give a concentration of 0.125 mol/L.Reaction occurred immediately after mixing, so that black particles wereprecipitated in a liquid. The particles were washed with ethanol andwere then dried, whereby the compound of Comparative Example 1 wasobtained.

Measurement of Absorbance

The compounds of Example 1 and Comparative Example 1 were measured forabsorbance, followed by determining the band gap of a light-absorbingmaterial from an obtained absorption edge.

FIG. 3 is a graph showing an absorption spectrum of the compound of eachof Example 1 and Comparative Example 1. In FIG. 3, the horizontal axisrepresents the wavelength of light and the vertical axis represents theabsorbance. The absorption spectrum of the compound of Example 1 isdrawn with a solid line. The absorption spectrum of the compound ofComparative Example 1 is drawn with a broken line.

From results shown in FIG. 3, it can be confirmed that the compound ofExample 1 has an absorption edge greater than that of the compound ofComparative Example 1.

The band gap of the compound of Example 1 was derived from the results,resulting in 1.45 eV. Likewise, the band gap of the compound ofComparative Example 1 was derived, resulting in 1.58 eV. Thus, it becameclear that the compound of Example 1 had a band gap smaller than that ofthe compound of Comparative Example 1 and was capable of absorbing lightwith a longer wavelength.

It is noted that the inventor repeated the preparation and analysis of acompound by the same methods as the above, while only the results of thecompounds of the Example 1 and Comparative Example 1 are shown herein.As a result, it became clear that compounds prepared by mixing asolution of the Cs source with the Ge-containing solution could have aband gap of 1.45 eV or more and 1.48 eV or less, with variationsdepending on a synthesis environment and a synthesis time. On the otherhand, it became clear that compounds prepared by mixing a powder of theCs source with the Ge-containing solution had a band gap of 1.54 eV ormore and 1.60 eV or less, with variations depending on a synthesisenvironment and a synthesis time. Thus, it was confirmed that a band gapsmaller than that of conventional CsGeI₃ could be obtained by preparingthe Cs source solution.

Measurement of X-Ray Diffraction

The compounds of Example 1 and Comparative Example 1 were measured byX-ray diffraction using Cu Kα radiation.

FIG. 4 is a graph showing an X-ray diffraction pattern of the compoundof each of Example 1 and Comparative Example 1. In FIG. 4, thehorizontal axis represents the diffraction angle (2θ) and the verticalaxis represents the diffraction intensity. The X-ray diffraction patternof the compound of Example 1 is drawn with a solid line. The X-raydiffraction pattern of the compound of Comparative Example 1 is drawnwith a broken line.

As shown in FIG. 4, the X-ray diffraction pattern of the compound ofExample 1 has a first peak a1 which is located at a diffraction angle(2θ) of 25.4° or more and 25.8° or less and which is assigned to the(111) plane of orthorhombic CsGeI₃, a second peak a2 which is located ata diffraction angle (2θ) of 24.9° or more and 25.3° or less and which isassigned to the (111) plane of rhombohedral CsGeI₃, and a third peak a3assigned to the (−111) plane of rhombohedral CsGeI₃. The intensity ofthe first peak a1 is about 68% of the intensity of the second peak a2.This result shows that in the compound of Example 1, orthorhombic CsGeI₃and rhombohedral CsGeI₃ are present in a mixed state. A relatively largepeak a4 detected at a diffraction angle (2θ) of about 30° is due tounreacted CsI.

On the other hand, the X-ray diffraction pattern of the compound ofComparative Example 1 has a second peak b2 which is located at adiffraction angle (2θ) of 24.9° or more and 25.3° or less and which isassigned to the (111) plane of rhombohedral CsGeI₃ and a third peak b3assigned to the (−111) plane of rhombohedral CsGeI₃ and lacks a peak(first peak) at a diffraction angle (2θ) of 25.4° or more and 25.8° orless. Thus, it is confirmed that the compound of Comparative Example 1contains rhombohedral CsGeI₃ and does not contain orthorhombic CsGeI₃.

A light-absorbing material according to an embodiment of the presentdisclosure is useful as a material for use in light-absorbing layers ofsolar cells. The light-absorbing material can be applied to a materialfor use in devices interconverting light and electricity.

What is claimed is:
 1. A light-absorbing material comprising a compound,wherein the compound has a perovskite crystal structure represented bythe formula AMX₃ where a Cs⁺ ion is located at an A-site, a Ge²⁺ ion islocated at an M-site, and I⁻ ions are located at X-sites, and at least apart of the compound has an orthorhombic perovskite crystal structure.2. The light-absorbing material according to claim 1, wherein an X-raydiffraction pattern of the compound measured using Cu Kα radiation has afirst peak at a diffraction angle (2θ) of 25.4° or more and 25.8° orless and a second peak at a diffraction angle (2θ) of 24.9° or more and25.3° or less, and an intensity of the first peak is 30% or more of anintensity of the second peak.
 3. The light-absorbing material accordingto claim 1, wherein the compound has a band gap of 1.35 eV or more and1.53 eV or less.
 4. The light-absorbing material according to claim 3,wherein the compound has a band gap of 1.45 eV or more and 1.48 eV orless.
 5. A solar cell comprising: a first electrode having electricalconductivity; a second electrode having electrical conductivity; and alight-absorbing layer between the first electrode and the secondelectrode, the light-absorbing layer converting incident light intoelectric charge, wherein the light-absorbing layer contains thelight-absorbing material according to claim
 1. 6. A method for producinga light-absorbing material including a compound having a perovskitecrystal structure represented by the formula AMX₃ where a Cs⁺ ion islocated at an A-site, a Ge²⁺ ion is located at an M-site, and I⁻ ionsare located at X-sites, the method comprising: (A) preparing a firstsolution containing Ge²⁺ ions and I⁻ ions; (B) preparing a secondsolution containing Cs⁺ ions; and (C) precipitating the compound byintroducing the second solution into the first solution adjusted to apredetermined temperature.
 7. The method according to claim 6, whereinthe (B) includes preparing the second solution by dissolving a Cs sourcein an organic solvent.
 8. The method according to claim 6, wherein the(A) includes preparing the first solution by mixing a Ge source,hydriodic acid, and phosphinic acid.
 9. The method according to claim 8,wherein the Ge source is germanium diiodide and the predeterminedtemperature is 50° C. or more and 100° C. or less.
 10. The methodaccording to claim 8, wherein the Ge source is germanium oxide and thepredetermined temperature is 20° C. or more and 50° C. or less.
 11. Themethod according to claim 8, wherein the (A) includes preparing a thirdsolution by dissolving the Ge source in an organic solvent, andpreparing the first solution by mixing the third solution with a liquidmixture containing the hydriodic acid and the phosphinic acid.